The optimal tilt for solar panels: why most calculators are wrong

To generate the most power, the advice is to have them perpendicular to the sun, but this isn’t always right.

Illustration of the optimal tilt for solar panels

If you’re a solar homeowner, of course you want your solar panels to generate as much electricity as possible. One of the ways to do this is by tilting them at the optimal angle so they capture the maximum amount of sunlight year-round.

So what is the best tilt angle for solar panels in the United States?

Short answer: a tilt of approximately 25° to 35° will work best for locations in the US. Lower tilts (closer to 25°) will be better for southern states, while those in the north will want to be closer to 35°. In reality, accurately determining the ideal tilt for your location is more complicated, and depends not just on your latitude, but the local climate, such as average cloud cover and even particulate matter from pollution.

For this reason, the advice that you can find elsewhere on the internet - such as making your solar panel tilt equal to your geographic latitude in degrees (ie. if you live at 45° latitude, your panel tilt should be 45°) - are incorrect. To learn the details about out why this is, keep reading.

What is the tilt of your solar panels important?

First, let’s talk about the basics of solar panel orientation and why it’s an important concept to understand.

When you’re making an investment in solar panels, you want them to generate as much electricity as possible. In a perfect world, we’d accomplish this by picking everything up and moving to a nice, sunny place like Hawaii. Of course, that’s not a viable option for most people, but as a Plan B, there are a few other things you can do to improve your solar electricity generation.

One of these is to optimize the orientation of your solar panels. When describing the orientation of your panels, there are two numbers: azimuth, which is the cardinal direction (north, east, south, or west) they are pointed and tilt, which is their vertical angle. Both are described in degrees.

Azimuth is in the angle relative to map north, where 0° is north, 90° is west, 180° is south, and 270° is east.

Tilt is the vertical angle, where a 0° tilt means that the panel is laying flat, and a 90° tilt means that it’s vertical.

To generate the most electricity, you would tilt the solar panel so that it’s perpendicular to the light source. I know that middle school was a long time ago, so I’ll clarify that perpendicular means a 90° angle. That means your solar panel would look like this: The optimal tilt for solar panels is perpendicular to the light source.
The optimal tilt for solar panels is perpendicular to the light source.

Okay, but how exactly do you keep solar panels pointed at the sun? Well, automated trackers do exist. Two-axis trackers use motors to tilt and rotate panels so they stay perpendicular to the sun as it moves through the sky during the day, but they’re very expensive. Cheaper one-axis trackers that change only the panels’ azimuth or tilt exist, but even those are often still too expensive.

In fact, only about 7% of large scale solar farms use any type of tracker, and very few residential installations do. Instead of spending money on an expensive tracker to try to squeeze out more performance from your panels, it’s almost always more cost effective to use fixed mounts and simply add more panels.

Tilt angle for roof-mounted solar

For this reason, most installations are fixed-tilt, meaning they remain stationary throughout the year. Roof-mounted solar is fixed tilt.

If you have a sloped roof, the panels will be mounted parallel and a few inches away from the roof deck. There aren’t solar racking systems for sloped roofs that allow the tilt to be adjusted. This means that if your home has a sloped roof, like most homes do, you have to just go with whatever your roof slope is. It’s not like you would redesign your roof so that it has the ideal slope for solar panels!

For flat roofs, the racking systems are installed either with deadweight (ballast) holding them in place, or they are bolted to the roof deck. In either case, these racking systems do allow the tilt to be adjusted at installation time, with the use of different height “feet” underneath the panels.

If you have a flat roof, keep in mind that the tilt angle that your panels will be kept at will probably be quite shallow. This is because a tilted solar panel will cast a shadow, and a more steeply tilted panel will cast a longer shadow. A flat roof on a house often may not have enough square footage to accommodate the larger separation required by steeply tilted panels, so you may need to accept a suboptimal tilt

Ground-mounted solar racking with adjustable tilt

If rooftop installations mean that you have little or no control over the tilt angle, does any of this matter to a homeowner?

It can. A small percentage (less than 5%) of homeowners choose ground-mounted systems. These racking systems sit in your yard, and are either pole-mounted racks or are racks supported by scaffolding. In either case, one big advantage of these systems is that the tilt angle can be controlled - in some cases, by the homeowner.

This means that if you own a ground-mounted system, instead of choosing a tilt angle that is based on the average position of the sun between summer and winter, you could change the tilt angle in summer and winter - or even more frequently - to really optimize your solar generation.

Ground-mounted systems are generally more expensive upfront, but they have a number of advantages. Being able to control tilt angle is one of them, but they also make it easier and cheaper to maintain and repair the system. Perhaps most significantly, with a ground-mount you don’t have to worry about the condition of your roof. Normally you want to have at least 15 years of life left in your roof before you install solar panels on them, because having solar panels on your roof means paying for their removal and reinstallation if you ever need a roof repair. With ground mounts, this is something you don’t have to worry about. To learn more, read our article about ground-mounted solar.

Most solar tilt calculators use a simplified model

Earlier, we mentioned that most solar tilt calculators you find on the internet are inaccurate. I’ll explain why.

The goal of choosing a tilt angle is to find the angle that keeps the solar panels as close to perpendicular to the sun throughout the year. The sun is higher in the sky in the summer than in the winter, so the optimal tilt for a fixed tilt system will be aimed at somewhere in the middle - the average of the sun’s position between the summer and winter extremes.

Solar tilt calculators are almost always based simply on the average position of the sun in the sky and your latitude (ie. your north-south position) on earth. By adjusting solar panel tilt to aim at the sun’s average position in the sky, you can theoretically maximize your solar production.

Unfortunately, this simplified model isn’t completely accurate because it only takes into account the sun as a source of light, but there are other significant sources of light: atmospheric scattering and clouds.

Sunlight scattered from clouds and the atmosphere

Have you ever seen pictures of the sun taken by astronauts in space or on the moon? There’s no blue sky: the sun is a blazingly bright spot in a backdrop of darkness. In this situation, a simplified model that treats the sun as the only source of light (as most solar tilt calculators do) is accurate.

But on Earth, we have a thick atmosphere. That’s why when we look up, we see a blue sky rather than a black one. As it turns out, that blue sky is a pretty strong source of light. Try this on a clear sunny day: look up at the northern sky without the sun in your field of view. It’s still pretty bright! You might even want your sunglasses.

Your solar panels will also “see” and benefit from that light. This is called atmospheric scattering, and it’s the reason why solar panels will still generate some electricity even if there isn’t direct sunlight.

In fact, you can point your solar panels north at an orientation where they never receive direct light from the sun, and they’ll still generate a decent amount of electricity on a sunny day. The power generation would certainly be less than if they were pointed south, but it can still be significant.

Atmospheric scattering and solar power

The Earth’s atmosphere normally causes light scattering, but the air isn’t always clear: pollution in the form of particulates and nitrogen oxides (which cause haze) can have a significant effect. This type of pollution can be caused by such things as fossil fuel power plants, forest fires, dust storms, and vehicle exhaust. For this reason, an accurate solar energy calculator needs to take into account both normal atmospheric scattering and the effect of pollution.

Pollution of this type can be quite predictable. In the United States, meterological stations measure common pollutants such as particulates, ozone, and nitrogen oxides, so this data is commonly available.

Because of that, it’s possible to include this data in a model that accurately takes into account both direct sunlight and atmospheric scattering, and calculate the ideal solar tilt angle for your location.

The Solar Nerd calculator and PVWatts

If you use The Solar Nerd calculator to estimate the cost of a solar panel system, the algorithm behind it takes into account climate conditions. This means that its tilt calculations use the more sophisticated approach of including both direct sunlight and atmospheric scattering.

However, the calculator is designed to make it easy to find out your cost of solar and - for the moment, at least - doesn’t give you a way to find out the ideal tilt for your location.

Fortunately, you can access the underlying algorithm yourself with a user-friendly interface. It’s called PVWatts, and it’s developed by the National Renewable Energy Laboratory. You can access it here:

When you use the calculator, it won’t tell you the ideal tilt right away. Instead, what you have to do is try different tilt values until you find the one that results in the maximum electricity generation. Setting the parameters for a PVWatts calculation (NREL)
Setting the parameters for a PVWatts calculation (NREL)

Once you set these parameters, click through to the next screen and you’ll see your estimated power generation for the year. For me, a tilt angle between 32° and 35° generates the maximum amount of electricity.

Bottom line and a secret: the tilt angle matters less than you might think

Using PVWatts is a neat and accurate way to calculate the optimal tilt angle for your solar panels, but you may also discover something else just as important: it probably doesn’t matter very much.

For example, when I set my tilt angle to 25° - a pretty large deviation from the optimal tilt - it results in only a 1% reduction in overall annual power generation.

Why is this? Buffalo is often fairly cloudy, which means that diffuse light plays a large role in the power generation for my system. Because the light is diffuse, it doesn’t matter as much if the panels are perpendicular to the sun.

It’s important to emphasize again that these effects are climate-dependent, so you should do your own calculation to find out if you get similar results in your location. But I bet that you will.

Here’s the major conclusion: if you do this calculation using PVwatts, you will probably find that it’s not worth spending extra money on any type of tiltable mounting system. Save your money and buy extra panels if you want more power.

The one possible exception? A flat roof, because these roofs can be quite shallow and too far out of alignment with the ideal tilt.

Further reading

The following academic paper by Stanford researchers Mark Z. Jacobson and Vijaysinh Jadhav provided much of the background for this article:

#Azimuth #Racking Systems

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